Conventional lithography encompasses a number of techniques. Photolithography is an important technique that is widely used for the fabrication of microstructures, such as semiconductor devices. Basically, it is usually performed as follows. First, a solution of photosensitive polymeric type of material dissolved in a solvent is spin-coated onto a substrate to form a homogeneous layer of a controlled thickness. Second, the substrate covered with the photoresist layer is heated to eliminate most of the solvent and the photoresist is selectively exposed to light using a patterned optical mask to block light where desired. Ultraviolet light is usually employed and its interaction with the photoresist material alters its physical or chemical properties in the exposed area in a manner sufficient to differentiate between exposed and non exposed areas of the photoresist during a subsequent development step. The photoresist can be of positive tone, wherein case it dissolves during the development step where it is exposed to light (i.e. the substrate can be etched in the exposed regions) or alternatively, it can be of negative tone and dissolve where it is not exposed to light (i.e. The substrate can be etched in the non-exposed regions). The photoresist, once patterned, acts as a protective mask to prevent local etching of the substrate. Once the image of the pattern is transferred into the substrate, the photoresist can be removed or stripped. Because, photolithography is usually performed under very clean and controlled environments, it is an expensive but powerful technique well suited for mass fabrication of structures and devices having lateral dimensions ranging from a few millimeters to less than 100 nanometers. The contrast and resolution of patterns made using photolithography should be as high as possible because it is not desirable to have some photoresist left in regions where the substrate should be etched away.
Other types of lithographic techniques follow the principle of patterning a resist to pattern a substrate but without using light. Electron-beam (e-beam) lithography, for example, uses a focused beam of electrons to write the desired image into an electron-sensitive resist. The image written in the resist is also developed in a subsequent step. In contrast to photolithography, e-beam lithography is not writing the image in the resist at once but it scans the beam over the resist to generate the elements of the pattern in a serial manner. E-beam lithography is consequently slower than photolithography but it has better resolution possibilities well below 100 nm. E-beam lithography is used for example for fabricating optical masks for use in photolithography. Electron beam resists can be of the negative or positive type. Positive e-beam resists are depolymerized by the beam of electrons and the written parts of the resist dissolve away during the development of the written image. Conversely, the regions of a negative e-beam resists which are written are polymerized and they are not dissolved away during the development step. Since e-beam lithography is a slow and expensive patterning technique, it is important to use either a positive type or negative type of resist depending upon the application to minimize the writing time. A pattern comprising a limited number of small structures is written faster using a negative type of resist because the writing operation produces a line of resist which can be transferred directly into the substrate. To fabricate the same line using a positive resist, it would be necessary to write a large area of the resist with the e-beam, omitting to write this line in the resist.
A technique called lift-off is an interesting strategy to invert the pattern of a developed resist. The lift-off technique can be used in photolithography and e-beam lithography. In lift-off, a resist is patterned first and a metal is then deposited over the patterned resist. The resist is subsequently removed together with the metal present on top of it. This technique is interesting because it can yield an overall negative process using a positive resist, but it has an inherent limited resolution and contrast. The fabrication of optical masks with e-beam lithography is not done using the lift-off technique for example.
There also exist other types of advanced lithographic techniques used or developed in research centers including those based on writing an image in a resist using X-rays or extreme UV light that are explored for achieving high resolution patterning of resists or for patterning very thick resists. However, among these advanced lithography techniques, the microcontact printing appears by far, one of the most promising and versatile.
Microcontact printing (MCP) utilizes an inked, micropatterned stamp to print chemicals or biomolecules onto a substrate. The most important application of MCP is the printing of alkanethiols onto Au, Ag or Cu substrates to form a self-assembled monolayer (SAM) in the regions of contact between the stamp and the substrate. The stamp is generally made from an elastomer such as polydimethylsiloxane (PDMS). PDMS polymers are commercially available under the trademark Sylgard (e.g. Sylgard 182, 184 and 186) manufactured by the Dow Coming Company, Midland, Mich. The PDMS stamp is replicated from a mold (typically a silicon wafer having a photoresist pattern formed thereon). The PDMS stamp is inked with a solution of SAM-forming molecules and dried to remove the solvent used to prepare the ink. The stamp is then placed onto the substrate to form a SAM in the printed regions of the substrate. It is possible to use the printed SAM as a patterned resist layer for selectively etching a substrate. In this case, the printed SAM protects the substrate from dissolution in an etch bath.
As a matter of fact, the material constituting the SAM when the latter plays the role of a resist layer, is quite different from conventional polymeric resists. First, a SAM forms spontaneously on different types of substrate and thus it does not need to be spin coated on a surface. For instance, the brief immersion of a gold substrate into a solution of alkanethiols diluted in ethanol leads to the formation of a SAM on the gold substrate. Second, a SAM is usually thin, about the length of one of its molecule or just a few nanometers thick. Finally, the etch system for patterning a substrate having a patterned SAM formed thereon must be carefully selected whereas conventional resists tend to protect well their substrates against many types of etchants. A relatively thin SAM can protect a substrate from dissolution in a wet etch bath provided that it has a good order and density over the substrate and that the etch bath is selective. A well-known example is the patterning of a gold substrate using a SAM of hexadecanethiol and a cyanide-containing etch bath. As an example, 0.5 ml of a 0.2 mM solution of hexadecanethiol in ethanol can be placed using a pipette onto the surface a 1 cm2 patterned PDMS stamp. The solution is left on the stamp for 30 s and then blown away with a stream of nitrogen. The stamp is dried with the stream of nitrogen and it is placed by hand onto the surface of a 20-nm-thick Au layer on a Si wafer. The contact between the stamp and the substrate enables the transfer of molecules of hexadecanethiol from the stamp to the substrate in the printed areas where the molecules chemisorb to the Au and form a SAM. A typical contact time is 10 s. The stamp is then removed by hand and the printed Au substrate is patterned using a selective wet etch bath: the printed SAM protects the Au from dissolution in an alkaline (pH of 12 or more) solution of water containing potassium cyanide and dissolved oxygen. After etching of the Au in the non printed regions, the patterned Au layer on the Si wafer is removed from the bath, rinsed with water and dried. Typical molecules for the ink are hexadecanethiol or eicosanethiol dissolved in ethanol. As a consequence, the materials forming such SAMs have recently sparked a lot of interest in the research community for their usage as high resolution resists: they have well defined chemical compositions, they are simple to prepare and of easier use than conventional resists. Finally, it is inexpensive to form SAMs on the surface of substrates and, moreover, SAMs can be patterned with a great variety of techniques.
Patterning of SAMs has been demonstrated using UV light and an optical mask to oxidize selectively molecules in a SAM. In these examples, the oxidized molecules lose their binding capability with the substrate so that they can be washed away from the surface in a subsequent rinsing step (see e.g. Tam-Chang et al., Langmuir 1996, vol. 11, p4371-4382).
Removing parts of a SAM with light and etching the substrate in the regions which are exposed to light correspond to a positive type of lithographic process. This work is severely constrained, however, by the instability of the SAM before it is photopatterned. This SAM is formed of short molecules which are sensitive to ambient light and unstable under standard laboratory conditions. Oxygen from air and stray light oxidize readily the sulfur moiety of the molecules in a monolayer, which lead to their easy removal in solution. This SAM has limited thermal stability, additionally. The lost molecules in the first SAM left regions of the substrate uncovered and resulted in the insertion of molecules of the second SAM. Since the second SAM forms the resist, it blocks the etch of the substrate in the regions where it is inadvertently inserted. This resulted in a poor contrast of the patterns of the etched substrate.
Patterning of SAMs has also been demonstrated by using the electrons of an e-beam microscope or of a scanning tunneling microscope to disrupt locally molecules in a SAM. The mechanism of interaction between the electrons and the molecules forming the SAM is not well known but the substrate can eventually be etched away in the written regions if an appropriate etch is used (see e.g. Lercel et al., J., Vac. Sci. Technol. B 1995, vol. 13, p1139-1143). In this case, the SAM material is used as a positive type of resist and the overall lithographic process is positive as well (the substrate is etched where the pattern is written). An attempt to pattern surfaces using an inverted process is done by Delamarche et al. (see e.g. Delamarche et al. J. Phys. Chem. B 1998, vol. 102, p3324-3334). In this work, the approach is similar to that of Tam-Chang et. but molecules forming the first SAM are removed using an electron beam instead of ultraviolet light. The same problems occur: the first SAM is too unstable under ambient conditions and tends to be exchanged by too many molecules during the formation of the second SAM in undesired areas.
Patterning a SAM has also been demonstrated on small length scales using mechanical indentation (see e.g. Abbott et al. Science 1992, vol. 257, p1380-1382). The blade of a scalpel or the tip of an atomic force microscope or of a scanning tunneling microscope can be used to damage and remove a protective SAM locally. An etching step can then transfer the written pattern into the substrate. The SAM forming material and the overall lithographic processes are of the positive type in this example. It can be desirable to employ an inverted process wherein a mechanical indentation would remove parts of a non-blocking etch SAM and to place an etch-blocking SAM in the indented areas. FIGS. 1a-1c shows the sequence of processing steps of the conventional method of patterning the surface of an object when the standard MCP process is used. As mentioned above, according to the standard MCP process, a SAM (playing the role of a masking resist layer) is printed on the object using a patterned rubber stamp.
Now turning to FIG. 1a, an object 10 comprises a substrate 11 coated by a layer 12 of a material to be selectively etched. The substrate 11 may be a silicon wafer and the layer 12 forming material can be made of gold (Au). A stamp 13 comprises a PDMS body fabricated from a mold to have a desired configuration, and a patterned layer 15 comprising an ink. Typically, the material forming the ink is eicosanethiol (ECT). The stamp 13 is applied to the object surface causing the formation of a self-assembled monolayer, also referred to as SAM 16 showing the same pattern as the printing area on the stamp 13. Finally, the object 10 is etched, for instance in a chemical bath to dissolve the gold at a location 17. The SAM 16 functions as a blocking in-situ resist layer to protect the gold layer 12 underneath. FIG. 1c shows that the gold layer 12 is etched everywhere it is exposed to the etchant from the bath. As apparent in FIG. 1c, the substrate 11 is protected where the SAM 16 was printed.
This standard MCP process has a few drawbacks and inherent limitations that will be discussed now.
Mechanical stability of the patterns on micropatterned stamps is a major problem. Stamps having small structures separated by long gaps (zones which should not come into contact with the substrate) are mechanically unstable. Gaps must be stabilized using supportive posts, or stamps must be fabricated using a harder elastomer. But stabilizing posts also print a SAM on the substrate and prevent etching the substrate in these regions. This can interfere with the layout of an electrical circuit or can create light absorbing zones where maximum transmission of light is desired, for example. Fabricating harder stamps might not be desirable in particular for printing on rough substrates or on substrates having already some structures present before printing because in these cases, the stamp would be too hard to deform enough to match well the topography of the substrate. It may be possible to fabricate a stamp with deepened structures to improve the stability of the gap regions of a stamp. However, in this case, a deep mold would be required, which might be difficult or expensive to fabricate. A reactive ion etcher is generally used to prepare such deep molds.
High-resolution lines on a stamp can be difficult to unmold, because lines and posts on PDMS stamps can break and detach when a stamp is peeled off a high-resolution master or if the structures are large but have a unfavorable geometry.
The time necessary for printing a SAM on a surface (of the order of a few seconds for 1-cm-large stamps up to several minutes for 15″-long stamps) is another important issue for technological applications of negative MCP. There are essentially two manners to place a stamp on a surface. In the first manner, the stamp is planar and brought gradually in contact with the substrate from one side, which ensures a controlled and well-propagating contact. This can be done by hand or using sophisticated tools. In either case, the stamp can be left in contact with the substrate for times ranging from seconds to minutes. Since one side of the stamp is placed first and removed last, the printing time can vary greatly from one side to the other. This results in different degrees of completion of the printed SAM, i,e. a side of the substrate will be more protected in the etch bath than the other and it would thus exist a gradient of varying protection from one side to the other. This can be another major problem for patterning a substrate because the SAM should be as homogenous and protective as possible everywhere. The second type of printing tool is using rolls or cylindrical stamps. It is relatively easy to have a homogeneous printing time with these tools but it is difficult to have long enough printing time to form well protective SAMs because the contact between the stamp and the substrate takes place only in a small area at a time.
In addition, it seems difficult, if not impossible, to print a molecule to form a SAM which is not compatible (swells, damages, cannot be inked, . . . ) with a stamp. This is unfortunate when the molecules forming the best monolayer on the substrate for a given application might not be forming a good or practical ink.
Finally, the above MCP process lacks flexibility. The standard MCP process described by reference to FIGS. 1a-1c, resembles to a negative type of lithography because the SAM plays the role of a resist layer which protects the substrate from etching in the regions where it is printed. It will thus be referred to hereinbelow as the negative MCP. Up to now, negative MCP has been used to print a SAM where the substrate needs to be protected, but obviously it could be also desirable to have the inverted process wherein the printed regions of the substrate would be etched away. In other words, it would be desirable to have the possibility of using SAMs for negative and positive types of lithography for many reasons, including reasons already known in conventional photolithography. It can be desirable to photopattern a SAM positively or negatively to obtain the best possible contrast and resolution for a specific pattern. It can be also advantageous to use one mask only to form positive or negative images of this mask by using a negative or positive type of SAM resist. It is further desirable to minimize the time necessary for writing a pattern in a SAM when slow and expensive patterning methods such as electron-beam lithography, atomic force microscopy, or scanning tunneling microscopy are used. The writing time in these techniques would be greatly decreased by using a positive or negative type of lithography using a SAM depending on the pattern.
In summary, the above described negative MCP process has the inconveniences and inherent limitations that are recited hereunder.    a. First of all, some micropatterned stamps are mechanically unstable because some of their features collapse during printing and areas of the stamp which should not come into contact with the substrate in fact print. To improve the stability, a mold with deep structures is fabricated to prevent sagging of parts of the stamp during the printing operation. Because, a reactive ion etcher is employed for this purpose, standard negative MCP processes are complex and expensive.    b. High-resolution lines on a stamp can be difficult to unmold.    c. Large substrates are long to print and the printing time is not homogeneous across the substrate. Consequently, with conventional negative MCP processes, SAMs printed over large substrates have different quality which leads to problems during the etch step subsequently performed.    d. It seems difficult, if not impossible, to print a thiol which is not compatible with a stamp material.    e. Finally, it would be highly desirable to have a positive MCP process for maximum flexibility.
No positive version of the above described standard negative MCP process has been known so far, although it would have been highly desirable to have both options like for any standard lithographic process. Such a novel approach of the MCP would not only correct some of the above drawbacks but would also offer a number of inherent advantages. A first result would be to have a maximum of flexibility for the fabrication of the mold and of the stamp, because it would be simpler and more economical to use directly a resist patterned on a substrate as the mold. Using a positive MCP process would solve the problem of stability of structures on stamps: large mechanically unstable gaps would become large printing areas and the areas of the substrate protected from etching would be the non printed areas. Finally, using a positive MCP process could solve the problem of having variable printing time (case of printing with planar stamps) or too long printing time (case of printing with a roll) by printing a SAM-forming compound which shall not be used as a resist but which shall block locally the formation of the resist-forming SAM in a subsequent step. Therefore, the SAM used as an etch resist could be patterned by depositing it in the regions complementary to the printed regions with optimal operating conditions on parameters such as time, solvent, concentration, temperature, and so forth.
The inventors have thus developed such a positive MCP process which not only solves the above mentioned problems of the standard negative MCP process but also opens new opportunities to the MCP technique. An application of this positive MCP process to an improved method of patterning the surface of an object will also be described.